Current demands for high density and performance associated with ultra large scale integration require design rules of about 0.18 microns and under, increased transistor and circuit speeds and improved reliability. As device scaling plunges into the deep sub-micron ranges, it becomes increasingly difficult to maintain performance and reliability.
Salicide processing involves deposition of a refractory metal that forms a compound with silicon, but does not react with silicon oxides, nitrides, or oxynitrides under normal processing conditions. Refractory metals commonly employed in the salicide processing include platinum, titanium, nickel, and cobalt, each of which forms very low resistivity phases with silicon, e.g., PtSi2, TiSi2, Ni2Si, and CoSi2.
In practice, the refractory metal is deposited in uniform thickness over all exposed upper surface features of a silicon wafer, preferably by means of physical vapor deposition (PVD). For example, the refractory metal can be sputtered by DC magnetron sputtering in an ultra-high vacuum, multi-chamber system. Tile formation of an MOS transistor requires the formation of a gate structure and source/drain junctions. The gate electrode typically formed by depositing a layer of heavily-doped polysilicon on a metal oxide insulating layer and etching the layers to pattern the electrode. Sidewall spacers are deposited on the opposing side surfaces of the patterned gate electrode. The sidewalls comprise silicon oxide, silicon nitride or silicon oxynitride. The MOS transistor further includes silicon oxide isolation regions formed in the silicon substrate between adjacent active device regions where the source and drain regions are formed or will be subsequently formed.
Generally, in forming the MOS transistor, the refractory metal is deposited after etching the gate electrode and after forming the source/drain junction. In a less common variant, the formation of the source/drain junction is effected subsequent to depositing the refractory metal layer via dopant diffusion through the refractory metal layer into the underlying semiconductor. In either case, after deposition, the refractory metal layer blankets the top surface of the gate electrode. As a result of thermal processing, e.g., a rapid thermal annealing process, performed in an inert atmosphere, the refractory metal reacts with underlying silicon to form electrically conductive silicide layer portions on the top surface of the polysilicon gate electrode and on the exposed surfaces of the substrate where source and drain regions are or will be formed. Unreacted portions of the refractory metal layer, e.g., on the silicon oxide, nitride, or oxynitride sidewall spacers and the silicon oxide isolation regions, are then removed, as by a wet etching process selective to the metal silicide portions.
FIGS. 1(A)-1(E) illustrate the are steps in a typical process in the manufacture of MOS transistors and CMOS devices for forming a nickel silicide layer. Nickel silicide has the advantage in that it possesses the same crystal structure with practically the same dimensions as silicon. Therefore, less mechanical stress will arise in monocrystalline silicon when nickel silicide is formed. In addition, nickel silicide has a great resistance to etchings which are used to etch silicon oxide. As a result, a silicon region having a top layer of nickel silicide may be readily provided with an insulating layer of silicon oxide which can be etched to form at least one contact hole for local contacting the nickel silicide regions.
Referring more particularly to FIG. 1(A), this figure shows a MOS transistor precursor 2. A portion of silicon semiconductor substrate 1 comprises a first conductivity type (p or n). It will be appreciated for P-MOS transistors, substrate 1 is n-type and for N-MOS transistors, substrate 1 is p-type. It is further understood that the substrate may comprise a plurality of n- and p-type regions arrayed in a desired pattern, as, for example, in CMOS devices. Precursor 2 is processed, as by conventional techniques which are not described here in detail, in order not to unnecessarily obscure the primary significance of the following description.
Precursor 2 comprises two isolation regions 3 and 3′ of silicon oxide, e.g., shallow trench isolation (STI) regions, extending from the substrate surface 4 to a prescribed depth below the surface. A gate insulator layer 5, typically comprising a silicon oxide layer about 25-50 Å thick, is formed on substrate surface 4. Gate electrode 6, typically of heavily-doped polysilicon, is formed over a portion of silicon oxide gate insulator layer 5, and comprises opposing side surfaces 6′ and top surface 6″. Blanket layer 7 of an insulative material, typically silicon oxide, silicon nitride, silicon oxynitride or silicon is then formed to cover all exposed portions of substrate surface 5 and the exposed surfaces of the various features formed thereon or therein, inter alia, the opposing side surfaces 6′ of gate electrode 6, the top surface 6″ of gate electrode 6, and the upper surface of STI regions 3 and 3′. The thickness of blanket insulative layer 7 is selected so as to provide sidewall spacers 7′ of desired width on each of the opposing side surfaces 6′ of the gate electrode 6.
Referring now to FIG. 1(B), the MOS precursor structure is then subjected to an anisotropic etching process such as by reactive plasma etching utilizing a fluorocarbon- or fluorohydrocarbon-based plasma comprising argon and at least one reactive gaseous species selected from CF4 and CHF3, for selectively removing the laterally extending portions of insulative layer 7 and underlying portions of the gate oxide layer 5, whereby sidewall spacers 7′ of desired width profile are formed along the opposing side surfaces 6′ of gate electrode 6. According to conventional processing, the entire thickness of the selected portions of insulative layer 7 and any underlying portions of gate oxide layer 5 are removed during the plasma etching process. Endpoint monitoring of the plasma etching process is typically achieved spectroscopically, as by loss of a characteristic oxygen peak of the plasma atmosphere upon complete consumption of the blanket insulative layer 7 and/or the gate oxide layer 5.
In FIG. 1(C), moderately to heavily-doped source and drain junction regions 8 and 9 of conductivity type opposite that of the substrate (or epitaxial layer on a suitable substrate) are formed in substrate 1 by conventional ion implantation (the details of which are omitted for brevity), with sidewall spacers 7′ acting as implantation masks and setting the lateral displacement length of moderately to heavily doped regions 8 and 9 from the respective proximal edges of gate electrode 6.
With reference to FIG. 1(D), the thus formed moderately- to heavily-doped source/drain regions 8 and 9 are subjected to a conventional high temperature treatment such as rapid thermal annealing to effect activation and diffusion of the implanted dopant species, thereby also forming lightly-doped, shallower depth source/drain extension regions 8′ and 9′ laterally extending from the respective proximal edges of the moderately- to heavily-doped source/drain regions 8 and 9 to just beneath the edge of sidewalls 6′ of gate electrode 6. It should be recognized, however, that the above-described method for forming source/drain regions including lightly-doped extensions is merely illustrative. Equivalently performing source/drain structures may be formed by alternative process schemes, for example, by first lightly implanting substrate 1 with dopant impurities of second conductivity type before sidewall spacer formation, with the implanted regions extending to just beneath the respective edges of the gate electrode, followed by selective heavy implantation of the lightly-doped implant (e.g., after sidewall spacer formation) to form heavily-doped source/drain regions appropriately spaced from the gate electrode by the lightly-doped (extension) implants.
With continued reference to FIG. 1(D), a blanket layer 10 of nickel is deposited by DC sputtering to cover the exposed upper surfaces of precursor structure 2. Following nickel layer deposition, the device is subject to rapid thermal annealing at a temperature and for a time sufficient to convert nickel layer 10 to electrically conductive nickel silicide where the nickel layer 10 is in contact with the underlying silicon. Any unreacted portions of nickel layer 10 such as those areas over silicon oxide isolation regions 3 and 3′ and silicon nitride sidewall spacers 7′ would be expected to be totally removed by a wet etch process. However, in the case where the sidewalls are silicon nitride, a very thin layer of nickel silicide 7A′ is formed on the outer surface of sidewall spacers 7′ as illustrated in FIG. 1(E).
While not wishing to be bound by any particular theory, the present inventors are of the opinion that the thin film of nickel silicide 7A′ forms as a result of reaction between the silicon in the nitride sidewall spacers and the nickel layer. Formation of the thin film of nickel silicide 7A′ on the outer surface of sidewall spacers 7′ disadvantageously results in the fonlation of a conductive bridge to the source, thereby shorting out the transistor.
Thus, there exists a need for an improved methodology for forming self-aligned silicide (i.e., salicide) contacts to ultra-thin transistor source and drain regions which provide greater device reliability and easy compatibility with conventional process flow for the manufacture of MOS-based semiconductor devices, e.g., CMOS devices. Moreover, there exists a need for an improved process for fabricating high quality MOS transistor-based devices which provides increased manufacturing throughput and product yield.